1. Field of the Invention
The present invention relates to surface acoustic wave devices (hereinafter, the term “surface acoustic wave” is abbreviated as SAW) such as SAW resonators, SAW filters, and duplexers, and more particularly, the present invention relates to a SAW device using shear-horizontal waves (hereinafter, abbreviated as SH waves).
2. Description of the Related Art
SAW filters have been widely used for band-pass filters of mobile communication devices and other electronic apparatuses. Such known SAW filters include SAW resonator filters and transversal filters.
An exemplary known SAW resonator filter is configured such that an ST-cut 0° X-propagation quartz substrate having excellent temperature characteristics has an interdigital transducer (hereinafter, abbreviated as an IDT) and reflectors disposed thereon. The IDT and the reflectors are both made from an Al electrode material, and use Rayleigh waves produced by excitation of the IDT.
Such a SAW device experiences the following problems:    (1) large losses due to a small electromechanical coupling coefficient; and    (2) difficulty in reducing the device size because of a small reflection coefficient, resulting in the need for a large number of fingers for reflectors in a device such as a SAW resonator and the SAW resonator filter.
Another exemplary known SAW filter is configured such that an ST-cut 90° X-propagation quartz substrate has an IDT and reflectors disposed thereon. The IDT and the reflectors are both made from an electrode material such as Ta and W and use SH waves produced by excitation of the IDT.
However, using an SH wave device having electrodes made from Ta or W gives rise to the following problems:    (1) difficulty in providing a SAW device which is available over a wide range of operating frequencies because of large losses due to a small electromechanical coupling coefficient and a large electric resistance of the electrodes; and    (2) difficulty in reducing the size of the SAW device because of a high acoustic speed.